Ring of Pain

You might get this randomly with items that stack up damage that overflows to multiple enemies. There are a few items that deal excess damage as soul damage or do damage to other enemies when you get hit or kill enemies. Stack enough and you might get lucky

The most easily reproducable method is probably the spell "Soul Spill" With this you can spend your total number of souls to do that much damage spead among all enemies. Towards the end of a run when you can amass souls in the hundreds, the spell can be enough to clear a room. So stockpile some souls, keep hold of Soul spill, and unleash it on a full room of 10+ enemies.
And it wouldn't hurt to have the previously mentioned items that deal soul damage/overflow damage on kill just to make sure.